The present invention relates to a protection method for molded-plastic surfaces and, more particularly, to a surfaces protection method which is capable of providing a surface with high wear resistance and good weathering properties. A protecting film formed on molded-plastic surfaces by the protection method of the present invention has a high thermal shock resistance.
Plastic panels, which are light and not easily broken, have recently come into wide use in place of glass panels. For instance, transparent plastic panels produced from a synthetic organic polymer are now generally used in transportation vehicles such as trains and buses, optical instruments and building materials.
In spite of the above-described merits, transparent plastics have some disadvantages. For instance, plastic surfaces are susceptible to damages such as scratches, which reduce the transparency and damage the external appearance. Furthermore, when transparent plastics are exposed to ultraviolet light as in sunlight for a long period of time, they are likely to discolored.
Various attempts have been made to improve the wear resistance of transparent plastics. For example, it is known that it is possible to provide wear resistance to molded-plastic surfaces of, for example, acrylic resins and polycarbonate resins by coating them with a film forming material which is composed of a mixture of colloidal silica and hydrolyzable silane (see, e.g., Japanese Patent Publication Kokai Nos. 55-94971 and 59-68377). With respect to a molded-plastic surface of polyethylene terephthalate, however, if the above-described film forming material is used without any primer, the bonding strength is poor. Not all primers are suitable in all situations, for example, in the case of polythylene terephthalate. The bonding strength is not improved with the above-described film forming material by applying polyalkylmethacryrate as a primer. In contrast however, a primer consisting of an acrylic polyol and its hardening agent isocyanate compound may be used. Even where the correct primer is used, if the curing of the primer is insufficient, the solvent in a top coat material will penetrate the primer layer, which becomes opaque and lowers the commercial value of the molded-plastic article. On the other hand, if the curing is sufficient, there is no problem of opaquing the primer layer and the bonding strength between the primer layer and the coated film is improved, but then cracking and crazing are likely to be produced on the top coated film.